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Subject: Re: [boost] [core] Breaking change to boost::ref in 1.56
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-07-14 03:47:54

On Monday 14 July 2014 10:20:41 Peter Dimov wrote:
> Agustín K-ballo Bergé wrote:
> > As mentioned in the pull request, cases like the following:
> > int i= 0;
> > boost::reference_wrapper<int> r = boost::ref(boost::ref(i));
> >
> > were performing "accidental collapsing", as it triggers an implicit
> > conversion followed by a (possibly elided) copy construction.
> Thanks.
> This works because of another boost::ref 'feature', it returns a const
> reference_wrapper which allows the outer boost::ref to bind to it. That
> const return is probably a mistake, now that I think of it, but it's too
> late to change it now.
> In fact, of the four ref/cref combinations, three happen to work as if
> reference collapsing overloads are present.
> What makes this hard is that I think that Eric is right and that the
> reference collapsing overloads should never have been added to std::ref. It
> should be obvious that nobody would write code like the above on purpose;
> there's absolutely no point in calling boost::ref twice when one call would
> do. It's similar to something like
> int x = std::cref( 5 );
> which is also something that used to work and is broken now, and also
> something that nobody would write except by mistake.
> The motivation for the reference collapsing overloads must come from
> examples taken from real code in which ref somehow gets applied to something
> that already is a reference_wrapper but not to ref(x) directly, and the
> code needs the result to be a flattened reference_wrapper for some reason.
> So far I've been unable to think of such an example. I wonder if anyone has
> ever encountered it. This feature looks like a typical committee invention
> to me.

I think Boost.Proto is one of the examples where undesired reference recursion
happened. I say 'undesired' because I cannot see why somebody would want a
reference to a reference. If not because of performance, conceptually there
are no references to references in C++, and std::ref attempts to reflect that.

Now that Boost.Proto does reference collapsing, reverting the change will
break it again (I think). Do you think we could settle with a trait I proposed
earlier and a prominent note in 1.56 release notes?

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